# Radioactivity – Exercise – 1

1. Plutonium decays with half life time 24000 years. If Plutonium is stored after 72000 years, the fraction of it that remain :

(a) 1/2
(b) 1/4
(c) 1/8
(d) 1/12

Explanation
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2. A radioactive substance decays to 1/16th of its initial activity in 40 days. the half life of the radioactive substance expressed in day is :

(a) 4
(b) 5
(c) 10
(d) 20

Explanation
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3. The half life time of a radio active elements of x is the same as the mean life of another radioactive element Y. Initially they have same number of atoms, then :

(a) y will decay faster thean x
(b) x will decay faster then y
(c) x and y will decay at the same rate at all time
(d) x and y will decay at the same rate initially

Explanation
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4. A and B are two radioactive substance whose half lives are 1 and 2 years respectively. Initially 10g of A and 1g of B is taken. The time after which they will have same quantity remaining is :

(a) 3.6 years
(b) 7 years
(c) 6.6 years
(d) 5 years

Explanation
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5. What Percent of original radio active substance is left after 5 half life time :

(a) 3%
(b) 5%
(c) 6%
(d) 12%